The present invention relates to safety devices in general, and in particular to an electrical load safety circuit.
Industrial machines such as power saws and drills pose a major safety threat in the event of an electrical power outage. When a power outage occurs, it is difficult to maintain records as to whether a machine was or was not operating at the time the outage commenced. Accordingly, machines set to an xe2x80x9conxe2x80x9d position during an outage are often mistakenly believed to be set to an xe2x80x9coffxe2x80x9d position. These machines commonly cause harm to equipment being worked on and workers themselves when power is unexpectedly regained.
To counteract this problem, industrial machines are sometimes equipped with a xe2x80x9csafety circuitxe2x80x9d which assure that a circuit is set to an xe2x80x9coffxe2x80x9d position when power is lost. These safety circuits generally include sensing circuitry in communication with a control switch. The sensing circuitry senses when power is lost and trips the control switch to the xe2x80x9coffxe2x80x9d position when there is a loss of power. Thus, for the machine to be turned back xe2x80x9conxe2x80x9d after a power failure, the control switch of the machine must be manually set to an xe2x80x9conxe2x80x9d position.
The inventor has found that commercially available safety circuits are unnecessarily complex, requiring an unnecessarily large number of circuit components at unnecessarily high costs. The safety circuit described in U.S. Pat. No. 3,958,164, for example, includes two relay-coils controlling three relay contacts, and four mechanical switches. Other known safety circuits include complex and relatively expensive solid state devices for sensing power losses and for tripping control switches.
In addition, while prior art safety circuits require that control switches be manually moved to an xe2x80x9conxe2x80x9d position after a power failure, they are still susceptible to problems resulting from a control switch being accidently or mistakenly m moved to an xe2x80x9conxe2x80x9d position after a power failure but before power is regained. If a machine control switch protected with a prior art safety circuit is mistakenly set to an xe2x80x9conxe2x80x9d position after a power failure but before power is regained, the machine protected by the prior art safety circuit poses the same safety hazard in the event of an unexpected power regain as a machine not protected by any safety circuit.
There is a need for lower cost equipment load safety circuit comprising a reduced number of circuit components.
According to its major aspects and broadly stated, the present invention is a low cost equipment load safety circuit comprising a limited number of circuit components.
A safety circuit according to the invention controls operation of an equipment load. A manual control switch of the safety circuit is connected in series with the equipment load for controlling the flow of electrical current to the equipment load. The equipment load and control switch series combination are further in communication with a relay contact switch of the safety circuit, which, like the manual control switch controls the flow of electrical current to the equipment load. Arranged in this configuration the equipment load operates only if both the mechanical switch and relay contact are closed. If either the manual control switch or the relay contact are open, the equipment load does not operate.
In further aspects of the invention, the relay contact is controlled by a relay inductor coil which is arranged in the safety circuit so that the relay contact opens only if there is a loss of electrical power and closes only if electrical power is available and both the relay contact and the switch is open. Therefore, an opening and closing of the control switch prior to a power regain will not result in the relay contact closing. This safety feature prevents in all cases the immediate operation of the equipment load on the unexpected regaining of power, even where the manual control switch is mistakenly or accidently opened and closed prior to a power regain. According to the invention, an opened control switch (which may be opened prior to or after a power regain) must be manually closed subsequent to a power regain for an equipment load to operate.
The drop-out and pull-in voltages of the circuit relay are carefully selected to assure proper circuit operation. The drop-out voltage of the relay is established, and the circuit complementally arranged so that the voltage across the relay coils falls below the drop-out voltage only if there is a complete loss of electrical power. The pull-in voltage of the relay is established and circuit complementally arranged so that the voltage across the relay coil exceeds the pull-in voltage only if the control switch is open and electrical power is available. This assures that operating power can only be supplied to the equipment load if the control switch is actuated (either opened and closed or closed from an open position) subsequent to a power regain.
In a preferred embodiment of the invention, the safety circuit includes a capacitor properly sized and arranged in the circuit such that the capacitor in combination with the relay inductor coil form a series circuit operating at about the series resonant frequency of the circuit when the control switch and relay contact are open. With this design, the voltage across the relay coil is assured of reaching a level substantially higher with the control switch open and the relay contact open than with the manual switch closed and the relay contact open. Providing a relay coil inductor voltage that is substantially higher with the manual switch and relay contact open than with the manual switch closed and contact open allows the pull-in voltage of the relay to be selected within a wide margin of error, and allows the selection of a pull-in voltage that is substantially spaced apart from both the maximum and minimum voltage drops across the relay coil, thereby enhancing the circuit""s performance reliability.
These and other details, advantages and benefits of the present invention will become apparent from the detailed description of the preferred embodiment herein below.